Old fields increase habitat heterogeneity for arthropod natural enemies in an agricultural mosaic

Spatially heterogeneous agricultural landscapes can support high levels of biodiversity. However, our understanding of the biodiversity value of most landscape elements and their role in the farmland mosaic is limited. We assessed the potential of old fields, a common small-scale feature of farmland in the Cape Floristic Region (CFR), for maintaining arthropod natural enemy (NE) diversity. We also assessed how habitable they are to NEs occurring in remnant fynbos vegetation, compared to the dominant surrounding vineyards. Furthermore, we compared habitat preferences of hymenopteran parasitoids, which are mostly habitat and trophic specialists, with those of generalist predatory arthropods. NE abundance and richness was as high in old fields as in natural vegetation, and was significantly higher than in vineyards. Old fields provided high plant diversity and prey abundance which were positively correlated with NE diversity. Ten out of eleven species of conservation significance assessed here occurred in old fields, including Western Cape and South African endemics, rare and uncommon species. Old field assemblages complemented fynbos and vineyard assemblages, and contained many unique species. They also shared many more species with natural assemblages than did vineyards, representing a more habitable transformed landscape element than cultivated areas for most species. Parasitoids showed greater habitat specificity than predators, and are likely to benefit more from the positive effect of increased habitat heterogeneity. Our results show that small-scale non-natural landscape features can provide additional resources, increase compositional heterogeneity and may help soften the overall agricultural mosaic for arthropod NEs in the CFR. Understanding the value of different landscape mosaic elements is an important step towards a broader landscape-scale approach to farmland biodiversity conservation.